1. Field
Embodiments relate to a torque-based walking robot, which controls various walking attitudes, and a control method thereof.
2. Description of the Related Art
Research and development of walking robots which have a joint system similar to that of humans and coexist with humans in human working and living spaces is actively progressing. The walking robots are multi-legged walking robots having a plurality of legs, such as two or three legs or more, and in order to achieve stable walking of a robot, actuators, such as electric motors or hydraulic motors, located at respective joints of the robot need to be driven. As methods to drive these actuators, there are a position-based Zero Moment Point (hereinafter, referred to as ZMP) control method in which command angles of respective joints, i.e., command positions, are given and the joints are controlled so as to track the command positions, and a torque-based Finite State Machine (hereinafter, referred to as FSM) control method in which command torques of respective joints are given and the joints are controlled so as to track the command torques.
In the ZMP control method, a walking direction, a walking stride and a walking velocity of a robot are set in advance so as to satisfy a ZMP constraint. A ZMP constraint may be, for example, a condition that a ZMP is present in a safety region within a support polygon formed by supporting leg(s) (if the robot is supported by one leg, this means the region of the leg, and if the robot is supported by two legs, this means a region set to have a small area within a convex polygon including the regions of the two legs in consideration of safety). In the ZMP control method, walking patterns of the respective legs corresponding to the set factors are created, and walking trajectories of the respective legs are calculated based on the walking patterns. Further, angles of joints of the respective legs are calculated through inverse kinematics of the calculated walking trajectories, and target control values of the respective joints are calculated based on current positions and target positions of the respective joints. Moreover, servo control allowing the respective legs to track the calculated walking trajectories per control time is carried out. That is, during walking of the robot, whether or not positions of the respective joints precisely track the walking trajectories according to the walking patterns is detected, and if it is detected that the respective legs deviate from the walking trajectories, torques of the motors are adjusted so that the respective legs precisely track the walking trajectories.
However, the ZMP control method is a position-based control method and thus achieves precise position control, but needs to perform precise angle control of the respective joints in order to control the ZMP and thus requires high servo gain. Thereby, the ZMP control method requires high current and thus has low energy efficiency and high stiffness of the joints, thereby applying high impact to a surrounding environment. Further, the ZMP control method needs to avoid kinematic singularities in order to calculate angles of the respective joints, thereby causing the robot to take a pose with knees bent at all times during walking and thus to have an unnatural gait differing from that of a human.
On the other hand, in the FSM control method, instead of tracking positions per control time, a finite number of operating states of a robot is defined in advance, target torques of respective joints are calculated with reference to the respective operating states during walking, and the joints are controlled so as to track the target torques. Such an FSM control method controls torques of the respective joints during walking and thus requires low servo gain, thereby attaining high energy efficiency and low stiffness of the joints and thus being safe with respect to a surrounding environment. Further, the FSM control method does not need to avoid kinematic singularities, thereby causing the robot to have a natural gait with stretched knees similar to that of a human.
However, the FSM control method controls walking of the robot depending on the finite number of operating states, defined in advance, and thus does not achieve proper control of walking of the robot, thereby causing the robot to lose balance. Therefore, the FSM control method requires a separate balancing motion to keep the robot balanced regardless of walking motions of the robot. For this purpose, command torques allowing the robot to be stably balanced so as to perform the balancing motion of the robot need to be calculated. In order to calculate the command torques, a complex dynamic equation needs to be solved. Thus, the FSM control method is not substantially applied to a robot having legs with 6 degrees of freedom now.